This study investigates the role of photoperiod, the pineal gland, and melatonin (a antigonadotrophic hormone of the pineal gland) in reproduction and temperature regulation in Peromyscus leucopus. In this species photoperiodism is regulated by an endogenous timer. We have also examined the effects of melatonin injections given at different times of day. Injections given in the afternoon result in photoperiodic effects when compared with morning injections. These melatonin effects appear to act on specific hypothalamic regions in the brain, given the results from the immunohistochemical techniques. In this species, melatonin alters several characters associated with seasonal changes in reproduction and temperature regulation.